In devices which perform a desired operation by transmitting input torque from a rotational driving source such as a motor to an output mechanism, when the driving source is stopped, there are occasions in which it is desirable to block the transmission of reverse input torque from the output mechanism back to the input side.
An example of such an occasion is when a retention function is employed to prevent the position of the output mechanism fluctuating when the driving source is stopped. In this type of device, taking an electric shutter as an example, the input torque from the driving motor in either a forward or reverse direction is input to an opening and closing mechanism on the output side, which then performs the operation for either opening or closing the shutter, although if for some reason (such as a power failure or the like) the driving motor is stopped partway through the opening or closing operation, reverse input torque resulting from the descent of the shutter under its own weight is returned to the input side, resulting in the possibility of damage to the input side components. Consequently, a mechanism is required which holds the position of the shutter, and prevents the return of reverse input torque from the shutter to the input side.
Furthermore, in a construction in which a reduction gear is used to reduce the revolutions of a motor, the following problems may arise in those cases where, for some reason, torque is reverse input from the output side.
(a) In a case in which worm gearing is used as the reduction gear, then because rotation under reverse input is impossible with this type of worm gearing, a very large load is exerted on the worm wheel or the teeth of the worm. In particular, a very large thrust loading acts upon the worm. As a result, there is a danger of damage to the bearing supporting the teeth and the worm, or alternatively, the mechanism must be increased in size in order to prevent this type of damage.
(b) Even in the case of a reduction gear which utilizes a spur gear or a helical gear, there is still a possibility of damage to the teeth in those cases where the reverse input torque becomes excessively large (such as the case of a shocking reverse input).
In order to resolve the problems outlined above, a mechanism is required which is capable of transmitting input torque from the motor of the input side to the output side, but also capable of locking the output side with respect to reverse input torque from the output side, thereby preventing the return of reverse input torque to the motor or the reduction gear on the input side.
Furthermore in recent years, many vehicles including automobiles have been equipped with motor driven electric retractable door mirrors, wherein the mirror moves through an angle of approximately 90° between a working position in which the mirror protrudes out from the side of the vehicle, and a retracted storage position. A conventional electric retractable door mirror (such as that disclosed in Japanese Patent Laid-Open Publication No. Hei 11-51092) utilizes a driving mechanism such as that shown in FIG. 29, wherein a mirror 42 can be moved easily by driving a motor 41, but when an external force acts upon the mirror 42, a clutch 43 effectively blocks the external force, holding the mirror 42 firmly in place and preventing the external force from acting upon the motor 41.
However, in the driving mechanism disclosed in the above publication, because the mirror is securely fixed and undergoes no rotation even if an external force results in a reverse input torque acting upon the mirror, the mechanism is unable to absorb such an external force, and the mirror is consequently prone to damage. In order to resolve this problem, a mechanism is required which is capable of transmitting input torque from the motor of the input side to the mirror of the output side, but also permits the mirror to slip with respect to reverse input torque, thereby blocking the transmission of such reverse input torque back to the input side.